1. Krashlander is free, and is a ridiculously hard physics puzzle game in which you play a skier attempting to knock over robots. There are points where the difficulty level borders on QWOP-level difficulty, frankly, but it's addictive and a blast. I nabbed it last time it was free, and have no regrets.
2. Not free, but Road of Kings just came out, and one of the devs is married to one of my co-workers, so I at least want to mention it. It's a single-player adventure game overtly inspired by boardgames like Warhammer Quest and Talisman, in which you play a barbarian attempting to become king. It looks nifty, and I wish they'd released it on Wednesday, when most games drop, because I suspect it'll get a little overlooked.
3. Back to free, Tower of Fortune is an RPG/Slot Machine hybrid (really). It's cheesy fun.